Power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) having an upper and lower electrode structure is a semiconductor device used, for example, for power conversion of home electrical appliances, car motors or the like. Since this kind of the semiconductor device is used for power, it is required to have a high withstand voltage. Additionally, for this kind of device, it is necessary to decrease the resistance (hereafter, ON-state resistance) at ON-operation for lower power consumption.
There is a field plate configuration as a measure to decrease the ON-state resistance. In the field plate configuration, a gate electrode is provided in a trench via a gate insulating film, and a field plate electrode is provided under the gate electrode via a field plate oxidization film. When providing the field plate electrode, there is an advantage in that an impurity concentration in the drift layer of the MOSFET can be increased, and thus the ON-state resistance is reduced. Moreover, in this kind of MOSFET, the depletion of the drift layer can be made with ease by the field plate electrode, and the high withstand voltage is maintained.
However, with the miniaturization of the power MOSFET, trenches tend to have smaller pitches. As the pitches of the trenches become smaller, the base layer sandwiched between the trenches becomes narrower. As a result, it becomes more and more difficult to form the source layer to be provided on the surface of the base layer or a carrier extraction layer to be provided in the base layer. For example, the career extraction layer, for example, excludes holes generated by the avalanche breakdown. The avalanche resistance of the power MOSFET is improved by providing such layer. Therefore, a power MOSFET having low ON-resistance and high tolerance even when the miniaturization of the power MOSFET proceeds is required.